Revisit Times

Off-nadir viewing allows image collections on an extremely high revisit frequency. For instance, the older Landsat system did not allow for off-nadir viewing and was thus limited to revisit times on those occasions when the orbit carried the satellite back over the same location (i.e., over 2 weeks). New systems, with the ability to tilt the sensor view off-nadir, may have revisit times of an average of a few days. However, increased flexibility comes with the cost of degraded spatial resolution. For example, the Space Imaging (SI) systems can increase the look angle from nadir to capture an image at the same location every 1.5 days, but the G.P. will be degraded to 1.3 meters. If daily collection is required, then the GSD is degraded to 2.0 meters.

Off-nadir images also complicate the spatial accuracies of images, since scene elements will be displaced from their correct horizontal position due to topography. Fortunately, off-nadir viewing allows for the creation of stereo images and corresponding calculation of DEM that can be used to correct (or rectify) this distortion. For example, the SI has the ability to collect imagery in both a fore and aft direction along the orbital track. This feature will enable collection of high-quality stereo imagery within tens of seconds. This capability allows the SI system to create accurate DEM (90% LE) of 3.0 meters with ground control and 7.0 meters accuracy without ground control.

The increased number of satellite systems also increase the revisit time options. For instance, there are currently several vendors planning to launch high-resolution systems in the near future. If one vendor cannot capture an image of interest, it can be expected that another vendor will.